Various thermal transfer recording methods have hitherto been known in the art. In these thermal transfer recording methods, a thermal transfer sheet comprising a color transfer layer provided on a substrate sheet is image wise heated from its backside, for example, by means of a thermal head to thermally transfer the color transfer layer onto the surface of a thermal transfer image-receiving sheet, thereby forming an image.
The thermal transfer methods are roughly classified according to the construction of the color transfer layer into two methods, i.e., thermal dye sublimation transfer (sublimation-type thermal transfer) and thermal ink transfer (heat-fusion transfer). For both the methods, full-color images can be formed. For example, a thermal transfer sheet comprising colorant layers of three colors of yellow, magenta, and cyan or optionally four colors of yellow, magenta, cyan, and black is provided, and images of the individual colors are thermally transferred in a superimposition manner on the surface of an identical thermal transfer image-receiving sheet to form a full-color image.
The development of various hardwires and softwares associated with multimedia has led to the expansion of the market of the thermal transfer method as a full-color hard copy system for computer graphics, static images through satellite communication, digital images typified, for example, by images of CD-ROMs (compact disc read only memory), and analog images, such as video images.
Specific applications of the thermal transfer image-receiving sheet used in the thermal transfer method are various, and representative examples thereof include proofs of printing, output of images, output of plans and designs, for example, in CAD/CAM, output of various medical analytical instruments and measuring instruments, such as CT scans and endoscope cameras, alternative to instant photographs, output and printing of photograph-like images of a face or the like onto identification cards or ID cards, credit cards, and other cards, and composite photographs and commemorative photographs, for example, in amusement facilities, such as amusement parks, game centers (amusement arcades), museums, and aquaria.
The diversification of the applications has led to an increasing demand for the formation of a thermally transferred image on a desired object. One method proposed for meeting this demand comprises the steps of: providing an intermediate transfer recording medium comprising a substrate and a receptive layer separable provided on the substrate; providing a thermal transfer sheet having a dye layer; transferring the dye from the thermal transfer sheet to the receptive layer in the intermediate transfer recording medium to form a dye image on the receptive layer; and then heating the intermediate transfer recording medium to transfer the receptive layer with the image formed thereon onto an object (see Japanese Patent Laid-Open No. 238791/1987).
Sublimation transfer-type thermal transfer sheets can faithfully form gradational images, such as photograph-like images of a face. Unlike conventional images produced by printing inks, however, these images are disadvantageously unsatisfactory in durability (fastness or resistance properties), such as weathering resistance, abrasion resistance, and chemical resistance.
To solve this problem, a method has been proposed wherein a protective layer thermal transfer film having a thermally transferable resin layer is put on top of a thermally transferred image and the transparent thermally transferable resin layer is transferred, for example, by means of a thermal head or heating roll to form a protective layer on the image.
Further, Japanese Patent Application No. 41441/1999 describes a highly durable intermediate transfer recording medium comprising a receptive layer provided on a separable transparent substrate. In this intermediate transfer recording medium, after the formation of an image in the receptive layer, the receptive layer with the image formed thereon, together with the transparent substrate, is brought into contact with an object so that the image surface faces the object to transfer the image onto the object.
These conventional intermediate transfer recording media are useful from the viewpoint of improving the durability, but on the other hand, at the present time, the problem inherent in the use of the intermediate transfer recording medium has not been solved.
Specifically, the intermediate transfer recording medium is in many cases used in a roll form. In this case, a problem of blocking sometimes occurs between the sheet substrate side and the transparent sheet side in the medium. In particular, in the intermediate transfer recording medium wherein the transparent sheet portion including the receptive layer has been half cut, the image non-forming portion is removed from the half cut portion. When the intermediate transfer recording medium with the image non-forming portion removed there from is wound into a roll, blocking is likely to occur between the sheet substrate side and the transparent sheet side. This sometimes makes it difficult to rewind the roll. Further, in this intermediate transfer recording medium, since printing is carried out in a wider area than the area to be actually retransferred onto the object, the uppermost surface of the resin layer formed on the sheet substrate should be separable from the thermal transfer sheet.
In view of the problems of the prior art, the first aspect of the present invention has been made, and an object of the present invention is to provide an intermediate transfer recording medium which is free from the problem of blocking, is excellent in reparability at the time of rewinding of the roll, and has excellent reparability from the thermal transfer sheet at the time of printing.
Incidentally, the above protective layer should be partially transferred at the time of transfer by means of a thermal head or a hot roll and thus should be transferable. To this end, the protective layer should be a resin layer having a thickness of about several μm. This makes it impossible to impart fastness or resistance properties, such as high scratch resistance and chemical resistance, to images. Also for the protective layer formed in the intermediate transfer recording medium, satisfactory fastness or resistance properties, such as satisfactory scratch resistance and chemical resistance, cannot be imparted from the viewpoint of transferability. The formation of an image on an object using an intermediate transfer recording medium followed by lamination of a resin film onto the image on the object so as to cover the image is considered effective for the formation of the protective layer. In this method, however, when the object has a certain shape, cockling is likely to occur in the resin film upon the lamination. Further, the provision of an additional step such as treatment by a special device such as a laminator is necessary.
Japanese Patent Laid-Open No. 238439/2000 as a prior art technique discloses an intermediate transfer recording medium comprising: a support provided with a resin layer; and a transparent sheet provided with a receptive layer, the support provided with a resin layer having been stacked onto the transparent sheet provided with a receptive layer so that the resin layer is separable from the transparent sheet, the transparent sheet portion including the receptive layer having been half cut. The resin layer is a pressure-sensitive adhesive layer formed of a vinyl acetate resin, an acrylic resin or the like, an easy-adhesion layer formed of SBR, NBR or the like, or an EC layer formed of an extricable reins such as LDPE.
This prior art technique would suffer from the following problems.                1. Change in peel force with elapse of time: In the case of the material for the pressure-sensitive adhesive layer and the material for the easy-adhesion layer as the resin layer, the peel force between the resin layer and the transparent sheet is likely to increase with the elapse of time.        2. Lack of stable separation at the time of transfer: In the transfer of the transparent sheet from the intermediate transfer recording medium onto the object by means of a heat roll or the like with heating at about 100 to 200° C., the transparent sheet in the intermediate transfer recording medium cannot be stably separated.        3. Lack of reliability of the intermediate transfer recording medium from the thermal transfer sheet at the time of image formation: When the image to be formed on the receptive layer has been unfavorably off to the resin layer, or when an image, which is somewhat larger than the transparent sheet provided with the receptive layer, is printed for the formation of the image on the whole area of the intermediate transfer recording medium, fusing occurs between the thermal transfer sheet and the resin layer, resulting in breaking of the thermal transfer sheet.        
Accordingly, in a second aspect, an object of the present invention is to solve the above problems of the prior art and to provide an intermediate transfer recording medium, which can be used to form an image on an object, can form a protective layer on the image to fully impart fastness or resistance properties to the image, permits the protective layer to be transferred onto the image with high accuracy in a simple manner, and can realize stable release from a thermal transfer sheet at the time of image formation and stable separation from an object at the time of transfer onto an object.